1. Field of the Invention
The present invention relates to a zoom finder optical system, and more specifically, to a real-image zoom finder optical system for use in a lens shutter camera and the like.
2. Description of the Prior Art
In recent years, with the appearance of smaller-size, higher-zoom-ratio and wider-angle (because of the popularity of panorama-size photos) lens shutter cameras, similar specifications are required for finder optical systems used in the lens shutter cameras. Specifically, wide-angle, high-zoom-ratio (ranging from wide angle to telephoto) and small-size finder optical systems are required.
The conventionally used high-zoom-ratio finder optical system is a real-image finder optical system comprising from the object side a positive objective lens system, a positive condenser lens and a positive eyepiece system. As a zoom finder optical system of this type, an optical system has been proposed having an objective lens system provided with a special feature.
For example, with a zoom finder optical system of a type where the objective lens system is of positive, negative, positive configuration and zooming is performed by moving the second and third lens units (Japanese Laid-open Patent Application No. H3-87803), a compact finder optical system with a high zoom ratio is realized.
However, in view of the realization of a wide angle (corresponding to an angle of view of 28 mm) and a high zoom ratio (3.times. or higher), in an objective lens system whose first lens unit has a positive refractive power, although an appropriate refractive power arrangement is obtained paraxially, in actuality, the necessary effective aperture of the first lens unit is too large at the shorter focal length condition (wide angle condition), so that it is impossible to secure the necessary thickness of the edge of the positive lens.
Therefore, to realize a wide angle and a high zoom ratio, it is desirable for the first lens unit of the objective lens system to have a negative refractive power. As a zoom finder optical system of such a type, optical systems are known which are of negative, positive, positive configuration (Japanese Laid-open Patent Application No. H1-116616) or of negative, positive configuration where zooming is performed by moving the first and second lens units constituting the objective lens system.
Generally, lenses provide a light beam having a large angle of view with a large amount of aberration. In particular, distortion is generated in proportion to the cube of the angle of view. Therefore, since the angle of view is large at the shorter focal length condition, a large negative distortion is generated by the first lens unit. Conversely, at the longer focal length condition (telephoto condition), since the angle of view of the light beam incident on the first lens unit is small, distortion is hardly generated. For example, in the case of 3.times. zoom, the angle of view is also tripled, so that distortion generated by the first lens unit at the longer focal length condition is 1/27 times that at the shorter focal length condition.
Because of this, in the optical systems of the type having the objective lens system of negative, positive, positive configuration (Japanese Laid-open Patent Application No. H1-116616) or of negative, positive configuration, a large negative distortion is generated by the first lens unit due to the increase in angle of view at the shorter focal length condition, so that the difference between distortion at the shorter focal length condition and distortion at the longer focal length condition (hereinafter referred to as "distortion difference") increases to a impermissible level. Thus, if the realization of a wide angle and a high zoom ratio is intended with this type, a large negative distortion is generated by the first lens unit only at the shorter focal length condition, so that aberration correction is difficult.
The present applicant disclosed in Japanese Laid-open Patent Application No. H4-51108 a zoom finder optical system of a type where the objective lens system is of negative, negative, positive configuration and zooming is performed by moving the second and third lens units.
This negative, negative, positive type objective lens system is of an arrangement where the refractive power is divided into two by dividing into two units the negative first lens unit of the negative, positive type objective lens system, so that the effect of distortion generated at the shorter focal length condition is restrained to reduce the distortion difference. This is because in the negative, negative, positive type objective lens system, the distortion generated by the first lens unit decreases since the refractive power of the first lens unit is small compared to the negative, positive type objective lens system. In addition, in the negative, negative, positive type objective lens system, since the angle of view is reduced by the first lens unit, the distortion generated by the second lens unit is also reduced.
As described above, the objective lens system of negative, negative, positive configuration can realize the wide angle. However, since the refractive power of the third lens unit increases too much if the high zoom ratio is realized, a large aberration is generated at the longer focal length condition where the light beam passes the outermost periphery of the lens (in other words, the aperture of the entrance pupil of the lens is large). In particular, spherical aberration is generated in proportion to the cube of the entrance pupil.